Support declaration-only enums in DWARF reader.

Support declaration-only enums in DWARF reader.

This patch adds declaration-only handling enums to the DWARF reader.

	* src/abg-dwarf-reader.cc (string_enums_map): Define new
	convenience typedef.
	(read_context::decl_only_enums_map_): Define new data member.
	(read_context::{declaration_only_enums,
	is_decl_only_enum_scheduled_for_resolution,
	resolve_declaration_only_enums}): Define new member functions.
	(build_internal_underlying_enum_type_name)
	(build_enum_underlying_type): Factorize these functions out of ...
	(build_enum_type): ... here.  Detect a decl-only enum and flag it
	as such.  If the enum type is decl-only, then set its underlying
	type as decl-only as well.
	(build_enum_underlying_type): Mark the underlying type as
	artificial.
	(get_opaque_version_of_type): Make this handle enums as well.  So
	make its return type be type_or_decl_base_sptr, rather than just
	class_or_union_sptr as it used to be.
	(read_debug_info_into_corpus): Add logging to trace decl-only
	enums resolution.
	(build_ir_node_from_die): Detect when a suppression specification
	makes an enum opaque.  In that case, get an opaque version of the
	enum type by invoking get_opaque_version_of_type.  Note that
	get_opaque_version_of_type doesn't support returning opaque
	-- i.e, decl-only enum types -- yet, but this is going to be
	handled in a subsequent patch.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>
Signed-off-by: Giuliano Procida <gprocida@google.com>